Abstract Clostridium difficile infection (CDI) is the most common healthcare-associated infection (HAI), accounting for 12% of all HAIs. The Centers for Disease Control and Prevention has labeled CDI as one of three urgent health threats that require immediate and aggressive action, as CDI is responsible for more than 450,000 hospitalizations and 29,000 deaths per year. Patients over 65 years old are most commonly affected and have the highest mortality. C. difficile causes disease through production of toxins, and the increase in incidence, severity, and mortality is in part due to the spread of a hypervirulent strain (NAP/BI/027) that produces more toxin. Pre-existing colonization with toxigenic C. difficile is a risk factor for developing CDI. Use of antibiotics that disrupt the protective normal colonic flora is the single largest risk factor for both C. difficile colonization and CDI. Current strategies to prevent CDI involve infection control measures to prevent transmission of C. difficile and antibiotic stewardship to control antibiotic use. Ultimately, however, infection control cannot prevent cases of CDI in patients already colonized. Thus, new measures need to be developed to identify patients at risk for CDI due to toxigenic colonization and prevent progression to disease. To date, no studies have considered primary prophylaxis as an approach to preventing CDI in appropriate patients, despite the fact that prophylaxis is a universally accepted method of preventing other diverse infections. In this study, we propose to screen patients who are initiating a course of antibiotics considered high risk for CDI to determine who is colonized with toxigenic C. difficile and, therefore, at risk of developing CDI. We hypothesize that the most effective and affordable intervention in the colonized patient population is prophylactic treatment with vancomycin, which is efficacious against C. difficile. Our goal for this study is to implement a first-of-its-kind clinical trial with the following specific aims: 1) Determine the prevalence of toxigenic C. difficile colonization among the inpatient population treated with high-risk antibiotics and the incidence of CDI in these patients with respect to their state of colonization with C. difficile; and 2) Determine the effect of vancomycin prophylaxis on patients receiving high-risk antibiotics who are colonized with toxigenic C. difficile based on molecular testing. This clinical trial has the potential to cause transformative change in the methodology of preventing CDI and alter the standard of care when using high-risk antibiotics in the hospital setting. This is a significant contribution; this cost effective and easily implemented intervention has the potential to reverse the increasing incidence of CDI and associated mortality rates, decrease the economic burden associated with the disease, and improve patient quality of life with minimal discomfort and adverse impact to patients.